Such a gas analysis method and such a gas analyzer operate with laser spectroscopy, preferably based on infrared absorption, wherein a component of a gas sample is analyzed by the use of special methods for laser triggering and evaluation of the measured signals in order to find out whether this component, i.e., the given gas, is contained as a component in the gas sample and if so, at what concentration. The optical analyzer typically contains one or more lasers as radiation sources, optical elements for guiding the beam, as well as a sample cell and one or more radiation detectors.
A prior-art process for laser control and signal evaluation, as described, e.g., by M. Kroll et al. in Appl. Phys. Lett., Vol. 51 (18), pp. 1465-1467 ("Measurement of gaseous oxygen using diode laser spectroscopy"), pertains to the gas spectroscopic measurement of oxygen concentrations on the basis of the radiation absorption in the wavelength range of 760 nm to 770 nm (near infrared). The laser diode with a monitor diode type ML 440S from the manufacturer Mitsubishi, which is used as the radiation source there, is supplied with a control current that is composed of a d.c. component and an a.c. component with a frequency of 5 kHz. The working point of the laser diode is set with the d.c. component of the control current, while the a.c. component brings about a periodic sweeping in the range of the absorption lines. To achieve a possibly harmonics-free control of the laser diode, a sinus curve is selected for the alternating current. The laser diode and the monitor diode are mounted as a block on a thermostat-controlled surface, wherein the laser diode is brought to one of the known absorption lines of oxygen by varying the temperature of this surface.
After having passed through the gas sample, the radiation emitted by the laser diode reaches a detector means, which is connected to an evaluating circuit. The evaluating circuit consists essentially of a lock-in amplifier, with a signal input to which the measured signal of the detector means is supplied, and a reference input to which a signal voltage of the doubled a.c. component frequency of the laser diode is fed.
The evaluating circuit also contains a differential amplifier, which is likewise connected to the measured signal of the detector means and which receives a signal of the monitor diode, which signal is proportional to the radiation output from the laser diode. The output voltage of the differential amplifier corresponds to the absorption of the measured oxygen concentration. Since the absorption line is particularly weak in the case of oxygen, this so-called second harmonic of the absorption line, which corresponds to the output voltage of the lock-in amplifier, is used for the concentration measurement. The advantage of this process is that due to the detection with the doubled a.c. component frequency of the laser diode, the measuring frequency is shifted into a range in which the laser noise is markedly reduced. Furthermore, the great offset is eliminated, so that the dynamic range can be made more efficient for the evaluation.
A process for the laser spectroscopic determination of oxygen concentrations on the basis of a direct absorption measurement in the wavelength range of 760 nm to 770 nm has been known from U.S. Pat. No. 5,448,071. The laser diode with a model ML-4405 monitor diode from Mitsubishi, which is likewise used as the radiation source, is supplied with direct current varying in steps and having a periodic curve, wherein each period consists of a series of intervals with constant current. In one embodiment, the duration of each interval with constant current is about 0.1 msec to 10 msec. A phase-sensitive detection, as was described above, is not possible.
In this process, the base line is determined in a first step several line widths from the center of the absorption line by detecting the measuring radiation having passed through the gas sample, on the one hand, and a reference radiation, which does not pass through the gas sample, on the other hand. The base line thus determined is subtracted from the absorption signal of the line center, which is measured in a subsequent step. The base line drift, noise and/or interference effects can be eliminated by adjusting additional control circuits, so that the output signal corresponds only to a change in the signal in the gas sample and is thus proportional to the concentration.
Furthermore, to determine the center of the absorption line, the control current and thus the frequency of the laser are set and varied stepwise in such a way that the laser is led to a point of the ascending flank of the absorption line, to the center, and to a point of the descending flank of the absorption line, wherein the difference between the center and the point of the ascending flank is equal to the difference between the center and the point of the descending flank, i.e., the current intervals are equal. Should the mean current value not exactly correspond to the center of the line, the two points to the right and left of the center show different absorption signals. The control current will then be adjusted until the two signals to the right and left of the center become equal.
Based on this process, the points of the ascending and descending flanks are selected according to U.S. Pat. No. 5,491,341 such that their absorption signals correspond to half the absorption signal of the line center. The current intervals are then a measure of the width of the absorption line. Changes in the line width, as they occur, e.g., in the case of a change in the composition of the gas sample, can be compensated with this information.
One drawback of both the process described by M. Kroll et al. and the process according to U.S. Pat. No. 5,448,071 is that the use of the Fabry-Perot Laser diode ML-4405 does not permit long-term stable use, because mode jumps usually occur due to aging processes after an operating time of one year and the working point will no longer be located on the selected absorption line. Then, correct measurements are no longer possible. Tuning to another absorption line is not easily possible, because a stable mode must be found. Furthermore, the sensor must be recalibrated.
Furthermore, a great drawback of the process described by M. Kroll et al. is the fact that it is not possible to compensate changes in the line width of the absorption lines, which are caused by temperature and pressure variations as well as by collisions with different components of a gas sample (foreign gas effect), which lead to a distortion of the concentration measurement.